LTE standing for Long Term Evaluation is a currently used IP-based or packet switched radio access network of a mobile telecommunications network. The LTE network part is also called 4G radio access network as it followed a 3G network and provided a higher capacity and data rates compared to the 3G network. The 4G network comprises a 4G core network, the Evolved Packet Core, EPC, and the 4G radio access network, LTE, wherein LTE and EPC together form the Evolved Packet System, EPS. Currently the next generation of the mobile communications network is developed under the name 5G network with a 5G core network (5GC) and a new radio access network, also called new radio (NR). FIG. 1 shows different scenarios how these two networks may interact. In the scenario I a mobile entity 10 is only operating in the 5G network so that it is connected to a radio access node 20 of the 5G network and to a core network entity 30 of the 5G network. Scenario II is called “new radio assisted” scenario in which the mobile entity 10 registers via the radio access node 20 of the 5G network, however, the mobile entity is also connected to the LTE radio access node 21 and both access networks can be used for the user plane as indicated by the straight line connecting the mobile entity 10 to both radio access nodes. In the third scenario shown, the situation is similar compared to scenario 2, here the mobile entity 10 is connected via the LTE radio access node 21 to a 5G enabled core network node 31 of the 4G network, called the 5G enabled EPC (Evolved Packet Core).
In scenario IV the mobile entity 10 is connected via the LTE radio access node 21 to the 5G core network node 30 and in scenario V, which is called LTE assisted system the mobile entity registers via the LTE radio access node 21 to the 5G core network but both radio access nodes 20, 21 can be used for the user plane.
Another scenario not shown is the case where the mobile entity 10 is only operating in the LTE network and is thus connected to the LTE radio access node 21 which is connected to the evolved packet core of the network.
As can be deduced from the scenarios discussed above the LTE radio access node and the 5G radio access node can be connected to the 5G core network or 5G EPC. The LTE radio access node will also be connected to the evolved packet core partly to serve mobile entities, also called UEs (user equipments) hereinafter, but also mobile entities operating in the 5G network can use scenario 3 mentioned above were the new radio access node 20 is anchored in the evolved packet core and is supported as an additional data carrier to LTE.
Especially at the beginning of a deployment of a new network it cannot be always assured that data packet sessions requiring a certain quality of service such as a real-time data packet session for a voice or video call can be handled by the new network. In other situations the 4G or LTE network may not be able to provide the required quality of service or the required resources for such a call.
In current situations it is not possible that a mobile entity uses the IMS (IP multimedia subsystem) over a 5G radio access network which is connected to a 5G core network wherein the 5G radio access network does not support the required quality of service for the call. Hence, in deployments where LTE is providing the wider area coverage overlapping with 5G radio access network then also the 5G radio access network should support the required quality of service for a voice call or video call even though the voice service could be handled by the other of the two networks, here the LTE network. The lack of voice support in the 5G radio access network and 5G core network could imply that a voice centric mobile entity will always camp on LTE.